Nullable value types in C Sharp (C#) - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a nullable value type in C#?

A nullable value type is a value type that can also hold a null value, allowing it to represent the absence of a value. It is declared using a question mark after the type, for example, int?.

beginner

How do you declare a nullable integer in C#?

You declare a nullable integer by adding a question mark after the type: int? myNumber;. This means myNumber can hold any integer or null.

intermediate

What does the HasValue property do in nullable types?

The HasValue property returns true if the nullable type contains a non-null value, and false if it is null.

intermediate

How can you safely get the value of a nullable type?

You can use the Value property if HasValue is true, or use the null-coalescing operator ?? to provide a default value, for example: int value = myNumber ?? 0;.

intermediate

What happens if you try to access the Value property of a nullable type that is null?

Accessing the Value property when the nullable type is null throws an InvalidOperationException. Always check HasValue before accessing Value.

How do you declare a nullable double in C#?

A double!

B nullable double

C double?

D double*

In C#, you declare a nullable value type by adding a question mark after the type, like double?.

What does int? x = null; mean?

A x is a string

B x is a nullable reference type

C x is a non-nullable integer

D x is an integer that can be null

The int? means x is a nullable integer, so it can hold an integer or null.

Which property tells if a nullable type has a value?

A IsEmpty

B HasValue

C Value

D IsNull

The HasValue property returns true if the nullable type contains a value.

What does the expression myInt ?? 10 do?

A Returns myInt if not null, otherwise 10

B Always returns 10

C Throws an error if myInt is null

D Returns null if myInt is null

The null-coalescing operator ?? returns the left value if it is not null, otherwise the right value.

What happens if you access Value on a null nullable type?

A Throws InvalidOperationException

B Returns 0

C Returns null

D Returns default value of the type

Accessing Value when the nullable type is null throws an InvalidOperationException.

Explain what nullable value types are and why they are useful in C#.

Think about how you might represent an empty or missing number.

Describe how to safely work with nullable value types to avoid exceptions.

Consider how to avoid errors when the value might be missing.

Practice

1. What does a nullable value type in C# allow you to do?

easy

A. Store only non-null values

B. Store values only in reference types

C. Store multiple values at once

D. Store a value or no value (null)

Solution

1. Step 1: Understand nullable value types

   Nullable value types allow a value type to hold either a normal value or no value (null).

2. Step 2: Compare options

   Only Store a value or no value (null) correctly describes this behavior; others are incorrect because value types normally cannot be null.

3. Final Answer:

   Store a value or no value (null) -> Option D

4. Quick Check:

   Nullable value type = value or null [OK]

Hint: Nullable means value or null, not multiple values [OK]

Common Mistakes:

• Thinking nullable stores multiple values
• Confusing nullable with reference types
• Assuming nullable cannot hold null

2. Which of the following is the correct syntax to declare a nullable integer in C#?

easy

A. int number?;

B. int? number;

C. nullable int number;

D. Nullable number int;

Solution

1. Step 1: Recall nullable syntax

   In C#, you add a question mark after the value type to make it nullable, like int?.

2. Step 2: Check each option

   Only int? number; uses the correct syntax int? number;. Others are invalid syntax.

3. Final Answer:

   int? number; -> Option B

4. Quick Check:

   Nullable int syntax = int? [OK]

Hint: Use ? right after type for nullable, like int? [OK]

Common Mistakes:

• Placing ? after variable name
• Using 'nullable' keyword incorrectly
• Mixing order of type and nullable

3. What is the output of this C# code?

int? x = null;
int y = x ?? 5;
Console.WriteLine(y);

medium

A. 0

B. null

C. 5

D. Compilation error

Solution

1. Step 1: Understand the null-coalescing operator

   The operator ?? returns the left value if not null; otherwise, it returns the right value.

2. Step 2: Apply to given code

   Since x is null, y gets the value 5, so Console.WriteLine(y); prints 5.

3. Final Answer:

   5 -> Option C

4. Quick Check:

   x is null, so y = 5 [OK]

Hint: ?? returns right value if left is null [OK]

Common Mistakes:

• Assuming null prints as 0
• Confusing ?? with || operator
• Expecting compilation error for null assignment

4. Identify the error in this code snippet:

int? a = 10;
int b = a;
Console.WriteLine(b);

medium

A. Cannot assign nullable int to int without checking for null

B. Variable 'a' must be initialized

C. Console.WriteLine cannot print int values

D. No error, code runs fine

Solution

1. Step 1: Check assignment from nullable to non-nullable

   Assigning a nullable int directly to an int variable without checking for null causes a compile error.

2. Step 2: Explain why error occurs

   Because a might be null, C# requires explicit null check or use of .Value or null-coalescing operator.

3. Final Answer:

   Cannot assign nullable int to int without checking for null -> Option A

4. Quick Check:

   Nullable to int needs null check [OK]

Hint: Check nullable has value before assigning to non-nullable [OK]

Common Mistakes:

• Ignoring null check before assignment
• Assuming implicit conversion works
• Thinking Console.WriteLine causes error

5. Given a nullable double double? d = null;, which code safely assigns its value to a non-nullable double result with default 0.0 if d is null?

hard

A. double result = d ?? 0.0;

B. double result = d.Value;

C. double result = (double)d;

D. double result = d != null ? d : null;

Solution

1. Step 1: Understand safe assignment from nullable

   To assign a nullable to non-nullable safely, provide a default value if null using ??.

2. Step 2: Evaluate each option

   double result = d.Value; throws exception if d is null. double result = d ?? 0.0; uses ?? to assign 0.0 if null, which is safe. double result = (double)d; casts but throws if null. double result = d != null ? d : null; tries to assign null to non-nullable, invalid.

3. Final Answer:

   double result = d ?? 0.0; -> Option A

4. Quick Check:

   Use ?? to provide default for nullable [OK]

Hint: Use ?? operator to assign default if nullable is null [OK]

Common Mistakes:

• Using .Value without null check
• Casting nullable directly without check
• Assigning null to non-nullable variable